There are very many patent documents illustrating articulated-arm three-dimensional measurement apparatuses having a plurality of articulation axes. The most recent measurement apparatuses commonly have as many as six articulation axes.
In general, articulated-arm three-dimensional measurement apparatuses conventionally include angle encoders integrated in said arm for individually measuring the angles of rotation of each of the articulation axes.
By way of example, reference can be made to the following documents: U.S. Pat. No. 5,402,582, U.S. Pat. No. 5,611,147, U.S. Pat. No. 5,794,356, U.S. Pat. No. 5,829,148, and U.S. Pat. No. 5,926,782.
Such articulated-arm three-dimensional measurement apparatuses need to be extremely accurate, and whenever action is performed on one of the arms, it is always necessary to recalibrate the entire measurement apparatus.
A large amount of effort has recently been made in an attempt to improve such three-dimensional measurement apparatuses, in order to simplify their electrical connection means and minimize the weight of each articulated arm so as to achieve better comfort during manipulation by an operator.
Thus, for example, the angle encoders associated with the articulated arms of conventional measurement apparatuses used to give rise to a very large number of electric wires being necessary for providing the connections between each encoder and an interpolation card, and then between the interpolation card and a counter, and finally between the counter and general communications means. Nowadays, because of recent developments in this field, a small card is available that performs simultaneously the interpolation, counting, and connection functions, while making use of bus cabling that is limited to five electric wires per angle encoder.
It is also increasingly desired to be able to modulate the geometry of the articulated-arm three-dimensional measurement apparatus as a function of the type of article being measured, and as a function of the space in which said article is situated while measurements are being performed. In this respect, specialists in the field have sought to be able to modify the length of one or more of the articulated arms of the three-dimensional measurement apparatus. The ability to use a shorter arm or a longer arm allows the operator to select the length of arm that is best suited to the volume of the measurement space and to the requested measurement accuracy, by optimizing a compromise between length and accuracy, given that tolerances for the parts to be measured are generally tight for small dimensions and slack for large dimensions.
The problem of replacing an arm by another arm of different length has been addressed in the field of articulated handling robots. Thus, document U.S. Pat. No. 4,984,959 describes an articulated handling robot having arms that house rotary shafts interconnected by angle transmissions. Each arm is constituted by a hollow tube interposed between two arm segments associated with the angle transmissions, with fastening being via abutting flanges that are bolted together, thereby providing mechanical coupling in rotation with the internal shafts in question. Provision is indeed made for disassembly to enable one hollow tube to be replaced by another of a different length, but it is emphasized that this is done in the factory, and that thereafter it is necessary to perform general recalibration on the three-dimensional measurement apparatus. In any event, the couplings concerned are quite unlike those of a measurement apparatus in which the articulated arms incorporate angle encoders, such that that teaching is not transposable to articulated-arm three-dimensional measurement apparatuses of the above-specified type.
The Applicant has developed an articulated-arm three-dimensional measurement apparatus with an arm in segments, the segments being assembled to one another by bolting. Such an embodiment for the arm makes disassembly possible, thereby enabling one segment to be replaced by another segment of different length, but that can be done only in the factory, it being understood that it is then always necessary to perform general recalibration of the entire three-dimensional measurement apparatus. Thus, the possibility of performing disassembly remains heavily penalized by lengthy and difficult operations of disassembly, reassembly, and general recalibration, which operations must be performed in the factory.
The Applicant has also developed another articulated-arm three-dimensional measurement apparatus in which the arms are mechanically assembled by a system of three pins disposed at 120° and engaging in corresponding V-shaped grooves, thereby creating an isostatic configuration with six points of contact so as to avoid any need to recalibrate the apparatus as a whole. Nevertheless, such a structure has been found to be very sensitive to variations in clamping force, insofar as the contact points become flattened to a greater or lesser extent by the pins, thus leading to lack of rigidity in positioning, and consequently to geometrical disalignment. Furthermore, it has been found that the arm is not sufficiently rigid while the apparatus is in use, doubtless as a result of the fragility of the contact zones, thereby leading to inaccuracy which it is difficult to remedy.
There therefore exists a need for an articulated-arm three-dimensional measurement apparatus of structure that provides great flexibility in use with a minimum of constraints, and simultaneously optimizes matching between the accuracy that is requested and the volume of the measurement space.